1. Field of the Invention
This invention relates to floating vessels. More particularly, it relates to mooring systems for offshore vessels.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
When mooring offshore floating vessels and buoys, there is need for a method and apparatus to adjust the length and tension of the mooring lines. This need arises during initial installation to provide the correct geometry and pretension for the mooring system, and later in the life of the system, to account for changes to the system, wear, or creep in the mooring lines or anchor system. In certain situations, the adjustment of length must be performed over the lower section of the mooring, below a spring buoy. In some cases, the spring-buoy-to-vessel distance must be kept as initially deployed in order to preserve the proper function of the system.
One mechanism for performing these adjustments is described in U.S. Pat. Nos. 6,983,714 and 7,059,262 entitled Method and Apparatus for Offshore Mooring. These patents describe the use of a chain stopper/chain wheel to enable the motive force for line tensioning to be applied from a boat above, and to adjust the length of line above the preset portion of the line.
GB2484840 describes the use of a subsea chain jack on a subsea buoy. Tensioning apparatus is provided for tensioning a tether extending between a first structure and second structure. A support bracket is provided for attaching the apparatus with respect to the first structure. A tether holding arrangement is provided for securing the tether with respect to the apparatus. A pivotable articulating member having a tether receiving channel therethrough is provided, the receiving channel having a longitudinal axis substantially aligned with a tether departure axis. A support socket is adapted to pivotably receive the pivotable articulating member such that movement of the tether departure axis away from alignment with the receiving channel longitudinal axis results in corresponding pivotal movement of the pivotable articulating member with respect to the socket. A method of installing a production buoy using such tensioning apparatus is also described.
U.S. Pat. No. 5,934,216 describes a method and apparatus for tensioning and deploying mooring chain. A set of inboard and outboard pawls are provided in the tensioner/stopper device which may include a fairlead. The pawls are spaced and operate in a manner that at least one pair of pawls grabs the chain at any given time. This is said to prevent accidental loss of the chain overboard. The chain is tensioned as the inboard pawls are engaged to the chain and actuated hydraulically to pull the chain inboard. Pulling inboard allows the outboard pawls to slide over at least one link and lock into place behind that link. The inboard pawls are stroked outboard over the next link to be grabbed, with the outboard pawls engaging the chain, the inboard pawls slide outboard to obtain another grip on a subsequent link and the process is repeated to conclude the tensioning. For deployment, the outboard pawls are retracted while the chain is retained by the inboard pawls. The inboard pawls are stroked outboard to pay out the chain. At that time, the outboard pawls grab the chain for temporary support as the inboard pawls are repositioned for the next cycle.
U.S. Pat. No. 7,421,967 describes a mooring system for securing a floating vessel to the sea floor that comprises a plurality of mooring legs, at least one of which includes separate first and second mooring lines. The first mooring line comprises a first end which is connected to the vessel and the second mooring line comprises a first end which is secured to the sea floor. The mooring system also comprises a connection and tensioning device which includes a body, a bore which extends through the body, a chain stopper for adjustably securing the first mooring line to the body, and a connector for connecting a second end of the second mooring line to the body. In use, a second end of the first mooring line is inserted into the bore and the first mooring line is pulled through the bore while the body is subject to an opposing pulling force. Once the first mooring line is pulled through the bore a desired distance, the chain stopper maintains the first mooring line in position relative to the body to thereby secure the vessel to the sea floor.
U.S. Pat. No. 5,809,925 describes a chain stopper wherein a mooring chain is guided for movement through the frame of the chain stopper along a pair of upright rails, with vertical links of the chain received between the rails and horizontal links of the chain riding on top of the rails. A pawl is swingably mounted on the frame above the rails with inner legs of the pawl engaging a horizontal link of the chain at opposite sides of an adjacent vertical link. The pawl has outer legs which extend downward to a release pin. The release pin has grooves positioned to receive the bottom ends of the outer legs and prevent the pawl from moving in a direction which will allow loosening of the chain, unless the release pin is freed for rotation through an angle of about 90 degrees. The release pin is connected to a trigger assembly including a spinner block which is normally held against rotation by a trigger finger. Movement of the trigger finger frees the spinner block and thereby allows the release pin to move from a pawl-engaging to a pawl-released position. The force of the chain on the inner legs of the pawl swings the pawl automatically as the chain loosens by sliding along the rails. The spinner block rotates freely, with no mechanism restraining it or the release pin.
U.S. Pat. No. 4,862,821 describes a mechanism for tensioning a moving chain. In an anchoring system for a floating vessel which includes an anchor line comprising chain cable, a chain locker and a windlass having a chain wheel that conveys the chain cable during paying out from the chain locker, a mechanism is positioned between the chain locker and chain wheel to back-tension the chain during paying out. The mechanism has an axis along which the chain is passed with every second links oriented in a given plane. Paired brake shoes are positioned to either side of the plane and define braking surfaces of sufficient extent along the axis of chain movement that a given chain link and an immediately succeeding link of similar orientation can be simultaneously engaged during their movement to provide a continuous retarding effect. One pair of braking shoes is pivotally mounted on an appropriate support structure and urged with hydraulic cylinders towards the other pair thereby causing the brake shoes to engage the opposing faces chain link. The pressure of hydraulic fluid applied to the cylinders is adjusted to back-tension the chain sufficiently that sudden shocks to the windlass otherwise occasioned by tilting and jumping of chain links during conveyance over the chain wheel are avoided. Non-standard links and irregularities in the chain link surfaces such as weld lines are accommodated by contraction of the hydraulic cylinders and deflection of the pivoting brake shoes.
U.S. Pat. No. 4,936,710 describes a mooring line tensioning and damping system. The floating structure comprises one or more catenary mooring cables for anchoring the structure to the seabed. An extensible dynamic tensioner system is provided for maintaining a predetermined dynamic tension in each mooring cable, as the structure responds to cyclic wave forces, and for increasing the natural periods of oscillation of the pitch, roll, heave, surge, sway, and yaw motions of the moored floating structure by reducing the spring stiffness of the mooring system. A motion damping system is coupled between the dynamic tensioner system and the structure for damping the linear and angular displacements of the structure relative to the tensioned cables. The damping system selectively applies frictional forces against a movable member in the tensioner system. The movable member does not move relative to the cables.
U.S. Pat. No. 6,602,019 describes a device for fixing, tensioning or pulling an extensible traction element such as a cable. The device has two supports which can be moved in relation to each other in a transverse direction to the axis of the traction element. Several clamping jaws are mounted in displaceable fashion in pairs opposite each other on said supports. The clamping jaws have surfaces which grasp the traction element. When strain is placed on the traction element, the clamping jaws are displaced linearly at increasing distances except for the rear pair, in such a way that the clamping force can be evenly distributed over a great length, despite the extension of the traction element. This is said to allow, for example, steel cables with a high traction force to be tensioned without damaging the cable.
International Publication No. WO 2013/043049 describes a device for tensioning anchor chains, in particular mooring legs of off-shore vessels and installations, comprising a frame carrying connectors for holding together lower and upper portions of the chain to be tensioned. The frame further carries a tensioning mechanism for pulling at least one portion of the chain towards the other portion of the chain while the device is submerged.